College Chemistry : Molarity, Molality, Normality

Study concepts, example questions & explanations for College Chemistry

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Example Questions

Example Question #6 : Liquids And Solutions

A solution is prepared by dissolving  in  of water. The final volume of the solution is . Find the concentration of the solution in units of molality.

Possible Answers:

Correct answer:

Explanation:

Recall how to find the molality of a solution:

First, start by finding the moles of glucose that we have. The molar mass of glucose is .

Next, convert the grams of water into kilograms.

Now, plug in the moles of glucose and kilograms of water into the equation for molality.

Example Question #7 : Liquids And Solutions

A solution of hydrogen peroxide is  by mass. What is the molarity of the solution? Assume that the solution has a density of .

Possible Answers:

Correct answer:

Explanation:

Start by assuming that we have  of this solution. Recall that hydrogen peroxide has a molecular formula of .

Use the given density to find the mass of the solution.

Next, find the mass of the hydrogen peroxide present in the solution.

Convert the mass of hydrogen peroxide into moles of hydrogen peroxide.

Recall how to find the molarity of a solution:

Since we have  of solution, the molarity is .

Example Question #8 : Liquids And Solutions

What is the molarity of a solution in which  sodium hydrogen carbonate is dissolved in a  solution?  

Possible Answers:

Correct answer:

Explanation:

 sodium hydrogen carbonate dissolved in a  solution has 

The first step is to calculate how many moles of  are present.  

We calculate molarity with the following equation: 

Example Question #9 : Liquids And Solutions

How many milliliters of a  solution are needed to prepare  of ?  

Possible Answers:

Correct answer:

Explanation:

 of  solution are needed to prepare  of .  

We can use the formula 

Therefore, 

Example Question #10 : Liquids And Solutions

What are the concentrations of aluminum and sulfate in a 3.0 M solution of aluminum sulfate? 

Possible Answers:

Correct answer:

Explanation:

The relative concentrations of aluminum sulfate  are

Example Question #1 : Molarity, Molality, Normality

Which of the following choices is characteristic of molality?

Possible Answers:

Moles of solute per liter of solution

Useful in experiments with significant temperature changes

Equivalents per liter

Useful in experiments without significant temperature changes

Correct answer:

Useful in experiments with significant temperature changes

Explanation:

Molarity, molality, and normality are all units of concentration in chemistry. Molarity () is defined as the number of moles of solute per liter of solution. Molality () is defined as the number of moles of solute per kilogram of solvent. Normality () is defined as the number of equivalents per liter of solution. Molality, as compared to molarity, is also more convenient to use in experiments with significant temperature changes. This is because the volume of a solution increases with temperature, and heating causes molarity to decrease; however, since molality is based on masses rather than volumes, molality remains unchanged.

Example Question #2 : Molarity, Molality, Normality

What is the molality of a solution made by adding  of  to  of water?

Possible Answers:

Correct answer:

Explanation:

Molality () is defined as moles of solute per kg of solvent.

 is the solute (it is what is being dissolved) and water is the solvent (what is doing the dissolving).

 Let’s start converting  of  to moles by dividing by its molecular weight using scientific notion through the entire molality calculations as necessary to simplify calculations.

Now we must convert the  of water to  then divide   by it to get the molality:

Molality of solution: 

=

Example Question #3 : Molarity, Molality, Normality

What is the molarity of a solution containing  of solution containing  of ?

Possible Answers:

Correct answer:

Explanation:

Molarity  moles of solute per liter of solution.

We are given that there are  of  and  of solution.

First lets convert the  of solution to liters. It is easiest to use scientific notion in all calculations for easy simplification:

=

Now we must convert  of  to moles to get moles of solute per liter of solution.

This is done by dividing  by its molecular weight which is . Once again we can use scientific notion to simplify calculations:

  

Now we can divide   by  solution to get the molarity.

Example Question #4 : Molarity, Molality, Normality

How many  of water are needed to dilute    to 

Possible Answers:

Correct answer:

Explanation:

In order to solve for the volume of water needed we must use this equation:

solve for  since we are looking for the final volume of water needed to dilute the existing solution:

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